Isodose linesdefinition, influencing factors,

clinical application

LIBIN SCARIAResident Medical Physicist

Beams of ionising radiation have characteristic

process of energy deposition, in

order to represent volumetric &

planar variations in absorbed dose are

depicted by isodose curves

Dose distribution

PERCENT DEPTH DOSE

Quotient expressed as percentage, of the absorbed dose at any depth d to the absorbed dose at dmax along the central axis of the beam

Isodose curves– Lines joining the points of equal percentage depth

dose(PDD)– Curves are usually drawn at regular intervals of

absorbed dose & expressed as a percentage of dose

• Isodose charts– Family of isodose curves– PDD values are normalised at Dmax or reference

depth

Field size (dosimetrical):lateral distance between the 50% isodose lines at a reference depth

Field size (geometrical) : field defining light is made to

coincide with 50% isodose lines of the radiation beam projected on a

plane perpendicular to the beam axis and at standard SSD or SAD

Beam profile• Falloff of the beam– By the reduced side scatter– Physical penumbra width

• High dose or ‘horns’ near the surface in the periphery of the field

• Outside the geometric limits of the beam and the penumbra, the dose variation is the result of– side scatter from the field– both leakage and scatter from the collimator system

PENUMBRA• Dose transitions near the boarders of the

field• Region at the radiation beam over which

the dose rate changes rapidly as a function of distance from the central axis

Geometric penumbraTransmission penumbraPhysical penumbraScatter penumbra

For cobalt-60 teletherapy: P ~ 1cmFor linacs : P~ 3mm

Measurement of isodose curves• Ion chambers• Solid state detectors

– TLD,Silicon diodes,• Radiographic films• Computer driven devices

Ion chamber is the most reliable method due to its relatively flat energy response and precision

Beam Analysing System3D water phantom- IBA Wellhofer Blue Phantom

• Two ion chambers :– Detector A: to move in the tank

of water to sample the dose rate– Detector B: fixed at some point

in the field to monitor the beam intensity with time

• Final response A/B is independent of fluctuation in the output

Sources of isodose chartAtlases of premeasured isodose

chartsGenerated by calculations using

different algorithmsManufacturers of radiation generators

Parameters of isodose curvesBeam quality

Source size, SSD, SDD Penumbra

Collimation and flattening filterField size

Beam Quality• Depth of a given isodose curve increases with

beam quality• Greater lateral scatter associated with lower-

energy beams• For megavoltage beams, the scatter outside the

field is minimized as a result of forward scattering – becomes more a function of collimation than energy

4 MV 6MV 15MV

Field size One of the most important parameters in

treatment planning PDD increases as field size increases Field size dependence of PDD is less

pronounced for higher energy than for lower energies

Field size smaller than 6 cmRelative large penumbra regionBell shapeTPS should be mandatory for small field size

5*5 cm2 10*10 cm2 15*15cm2

SSD affects the PDD and the depth of the isodose curvesPDD icreases with SSD

Beam fattening filter• Intensity is more at central axis and

decreases as we move away• Non-uniform dose at any given depth• FF is used to uniform it• Usually made up of Al or Brass

Wedge filters Wedge shaped absorber causes a progressive

decrease in intensity across the beam Results in tilt of the isodose curve & degree of

the tilt depends upon the slope of the wedge filter

Material: tungsten, brass. Lead or steel

15o Wedge

30o Wedge

45o Wedge

60o Wedge

Wedge isodose angle (θ) is the complement of the angle through which the isodose curve is tilted with respect to the central ray of the beam at any specified depth

This depth is important because the angle will decrease with increasing depth.

Angle of isodose tilt to decrease with increasing depth in the phantom.

The choice of the reference depth varies:

10 centimeters1/2 - 2/3rd of the beam widthAt the 50% isodose curve

Hinge angle,φIt is the angle between central axes of two

beams passing through the wedge

Relationship b/w φ & θ

Wedge angle,θ= 90 – φ/2

Bolus• A tissue equivalent material

used to reduce the depth of the maximum dose (Dmax)

• In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions.

• The thickness is usually 0.5 cm to 1.5 cm

Combined Open Field Technique

• Criteria:– The dose distribution within the tumor volume is

reasonably uniform (±5%).– The maximum dose to the tissue in the beam is not

excessive (not more than 110% of the prescribed dose)

– Normal critical structures in the beam do not receive doses near or beyond tolerance

Parallel Opposed Fields

• Advantages– The simplicity and reproducibility of setup– Homogeneous dose to the tumour– Less chances of geometrical miss

• Disadvantage– Excessive dose to normal tissues and critical

organs above and below the tumour

Multiple fields To deliver maximum dose to the

tumour and minimum dose to the surrounding tissues

• Using fields of appropriate size• Increasing the number of fields or portals• Selecting appropriate beam directions• Adjusting beam weights• Using appropriate beam energy• Using beam modifiers

Multiple fields Certain beam angles are prohibited

• Presence of critical organs in those directions• Setup accuracy of a treatment may be better with

parallel opposed beam arrangement

The acceptability of a treatment plan depends not only on the dose distribution but also on

• Practical feasibility• Setup accuracy• Reproducibility of the treatment technique

Isodose lines of Electrons

Isodose curves• Scattering of electrons

determines shapes of isodose curves– Expansion– Lateral constriction

• Larger field size required at surface

Proton isoeffective dose

“Charged Particle Therapy” to better preserve healthy tissues and organs at risk

Medicine is

a science of uncertainity

and an art of

probabilityDr. William Osler

A canadian Physician and one of the four founding professors of John Hopkins Hospital,

USA

Thank u…